The present invention is in the field of inspection techniques of the kind utilizing irradiation of a sample by a focused beam of electrically charged particles, such as electrons, positrons, or ions, and relates to a deflection method and system, and a focusing/deflecting assembly utilizing the same, for use in a charged particle beam column.
Charged particle beam columns are typically employed in scanning electron microscopy (SEM), which is a known technique widely used in the manufacture of semiconductor devices, being utilized in CD metrology tools, the so-called CD-SEM (critical dimension scanning electron microscope) and defect review SEM. In an SEM, the region of a sample to be examined is two-dimensionally scanned by means of a focused primary beam of electrically charged particles, usually electrons. Irradiation of the sample with the primary electron beam releases secondary (and/or backscattered) electrons. The secondary electrons are released at that side of the sample at which the primary electron beam is incident, and move back to be captured by a detector, which generates an output electric signal proportional to the so-detected electric current. The energy and/or the energy distribution of the secondary electrons is indicative of the nature and composition of the sample.
SEM typically includes such main constructional parts as an electron beam source (cathode having a small tip called xe2x80x9celectron gun), an electron beam column, and a detection unit. The electron beam column comprises inter alia a beam aligning means (the so-called xe2x80x9calignment coilsxe2x80x9d), a beam shaping means (stigmator), and a focusing/deflecting assembly including a lens arrangement and a deflection system for directing a primary electron beam onto a sample and directing secondary electrons towards one or more detection unit. The deflection of the primary beam provides for scanning the beam within a scan area on the sample, and also for adjusting incidence of the primary beam onto the sample (an angle of incidence and/or beam shift).
Some systems of the kind specified utilize an objective lens arrangement in the form of a combination of a magnetic objective lens and an electrostatic lens, the so-called xe2x80x9ccompound magnetic-electrostatic lensxe2x80x9d (e.g., WO 01/45136 and EP 1045425 both assigned to the assignee of the present application, and WO 01/56056). The electrostatic part of the compound magnetic-electrostatic lens is an electrostatic retarding lens (with respect to the primary charged particle beam), and has two electrodes held at different potentials, one of the two electrodes being formed by a cylindrical anode tube which is arranged within a magnetic objective lens along its optical axis, and the other electrode being a metallic cup provided below the magnetic objective lens. A need for a retarding field is associated with the following. In an SEM, in order to reduce the xe2x80x9cspotxe2x80x9d size of the electron beam up to nanometers, a highly accelerated electron beam is typically produced using accelerating voltages of several tens of kilovolts and more. Specifically, the electron optic elements are more effective (i.e., produce smaller aberrations) when the primary electrons are accelerated to high kinetic energy. Generally, the landing energy of the primary electron beam is defined by the potential difference between the cathode (a source of primary electrons formed with a small tip called xe2x80x9can electron gun) and the sample. To achieve the desired acceleration of electrons, an appropriate potential difference between the cathode and anode (which is typically in the form of a tube defining a primary beam drift space for the primary beam propagation to the sample) should be provided. For example, the cathode voltage Vc can be about (xe2x88x921) kV and the anode voltage Va can be about (+8) kV. Hence, the electrons are accelerated on their way towards the magnetic objective lens having the velocities of 9 keV However, it has been observed that such a highly energized electron beam causes damage to resist structures and integrated circuits, and, in the case of dialectical samples, causes the undesirable charging of the sample. To avoid these effects, a retarding field is created in the vicinity of the sample. The electric field created by the electrostatic lens also facilitates the extraction of secondary charged particles from the sample.
The above-indicated publication WO 01/56056 also discloses the use of a magnetic deflector integrated into a magnetic objective lens, which has an excitation coil and upper and lower pole pieces. The magnetic deflector comprises excitation coils located on the lower pole piece of the magnetic lens, and the lower pole piece is divided into four pole piece segments, each segment having its corresponding additional excitation coil of the deflector. The additional excitation coils are wrapped around the pole piece segments of the magnetic lens, so that by exciting one the additional excitation coils, a magnetic field is generated in the corresponding segment of the lower pole piece. The magnetic field is basically perpendicular to the path of the electron beam (to the optical axis). Accordingly, a magnetic field across the path of the electron beam is generated which leads to a deflection of the electron beam. Due to the segments of the lower pole piece of the magnetic lens, the magnetic field is guided to an area close above the sample and generates the required strong deflection field. The segments of the lower pole piece of the magnetic lens at the same time also guide the magnetic field generated by the excitation coil of the magnetic lens.
There is a need in the art to improve the control of charged particle beam propagation through a lens arrangement in a charged particle beam column towards a sample under inspection, by providing a novel deflection method and system, and a lens arrangement utilizing the same.
The present invention is aimed at increasing the deflecting magnetic field at the optical axis of the lens arrangement in the vicinity of the sample""s plane at a given electric current through the excitation coils of the magnetic deflector, or obtaining a high magnetic field with a lower electric current through the excitation coils of the deflector. This allows for obtaining a desirably high deflecting magnetic field within the closest vicinity of the sample at the optical axis of the lens arrangement, without increasing a working distance, also in cases where the electrode of an electrostatic retarding lens is located between the magnetic objective lens and the sample.
The term xe2x80x9cworking distancexe2x80x9d is typically referred to as a distance between the electrode of the lens arrangement closest to the sample""s plane and the sample""s plane. This distance should be as small as possible, and the minimal possible working distance is typically defined by an arcing problem. The present invention provides for concentrating the magnetic deflecting field at the optical axis of the lens arrangement in the vicinity of the sample""s plane without affecting (increasing) the working distance, by providing a pole piece assembly at least partly located within the magnetic field of a magnetic deflector.
The problem solved by the present invention is associated with the following: To enable effective control of the magnetic field intensity in the vicinity of a sample (either grounded or not), an electrode closest to the sample should be formed with an opening as small as possible (e.g., of about 2 mm). Making the external pole piece of the magnetic objective lens with such a small opening will result in non-homogeneity of the magnetic and electric fields (due to the gaps between the pole piece segments of the magnetic objective lens or pole pieces of a magnetic deflector, as the case may be), and accordingly, in a distorted (blurred) image of the irradiated area of the sample. Using a larger inner diameter of the pole pieces is ineffective for both controlling the field intensity and deflection, since this requires a higher power supply resulting in undesirable heating of the pole pieces. In the above-described prior art constructions utilizing an electrostatic lens formed by the anode- and cup-electrodes, the reduction of the inner diameter of the magnetic objective lens or that of the deflector on the lens is limited by the funnel of the anode tube having a 15 mm diameter (under 8 kV and more voltage) inside the magnetic lens and the cup-electrode below the magnetic lens.
The present invention overcomes the above problem by providing the pole piece assembly, which conducts the magnetic field created by a magnetic deflector towards the optical axis of a lens arrangement (including a magnetic objective lens and optionally also an electrostatic lens). This increases the effectiveness of deflection by increasing a magnetic field for a given electric current through the excitation coils of the deflector (power supply). In this connection, it is important to note that it is often desired to operate with the so-called tilt mode, when a primary charged particle beam impinges onto a sample along an axis forming a certain angle with the sample""s surface. The tilt mode is usually utilized to inspect samples that have a surface relief, i.e., pattern in the form of a plurality of spaced-apart grooves, to detect the existence of a foreign particle located inside a narrow groove. The tilt mode 25 can be achieved by passing higher electric currents through the excitation coils of the magnetic deflector, as compared to those of a normal mode of operation (i.e., the primary charged particle beam impinges onto the sample with substantially zero angle of incidence). The present invention facilitates operation with the tilt mode, since it provides for increasing a deflecting magnetic field at a given electric current through the excitation coils of the deflector.
Thus, according to the invention, a magnetic deflecting field is created by a magnetic deflector having excitation coils and preferably pole pieces (generally, at least two pole pieces), and conducting the magnetic field of the deflector to the optical axis by a pole piece assembly at least partly located in the magnetic field of the deflector (having a magnetic contact surface with the pole pieces of the deflector). The pole piece assembly has a portion made of a soft magnetic material and is formed with an opening for a charged particle beam passage therethrough. The pole piece assembly, when mounted in the focusing/deflecting assembly of the column, is located such that the optical axis of the lens arrangement intersects with said opening.
The pole pieces of the magnetic deflector and the pole piece assembly are preferably electrically insulated from each other, and may be accommodated adjacent to each other or partly overlapping. The pole piece assembly may be constructed such that it has at least two pole piece elements partly or completely separated from each other by gaps. Alternatively, the pole piece assembly may be in the form of a single pole piece element (e.g., shaped like a disc) with the opening, in which case the pole piece assembly and the pole pieces of the deflector partly overlap each other.
Each of the at least two pole piece elements of the pole piece assembly may be separately operated by a voltage supply, thereby enabling beam scanning of the sample along at least one axis. By providing at least four pole piece elements in the pole piece assembly, scanning along two mutually perpendicular axes can be achieved.
Preferably, the pole piece assembly also comprises a central portion, which is formed with said opening and is made of a non-magnetic metal, and is surrounded by said portion made of the soft magnetic material. This central portion made of the non-magnetic metal may thus serve as the electrode of an electrostatic lens, the other electrode thereof being either an anode tube or the pole piece of a magnetic objective lens.
There is thus provided according to one aspect of the present invention, a deflection system for use in a lens arrangement of a charged particle beam column for inspecting a sample, the system comprising:
a magnetic deflector operable to create a magnetic field;
a pole piece assembly which has a portion made of a soft magnetic material and is formed with an opening for a charged particle beam passage therethrough, the pole piece assembly being accommodated so as to be at least partly located within the magnetic field of the magnetic deflector to thereby conduct at least a portion of the magnetic field created by the deflector through the pole piece assembly towards said opening.
According to another aspect of the present invention, there is provided an electrostatic lens for use in a lens arrangement of a focusing/deflecting assembly in a charged particle beam column for inspecting a sample, wherein the focusing deflecting assembly comprises a magnetic deflector, the electrostatic lens comprising an electrode that is formed with an opening for a charged particle beam passage therethrough and is made of a non-magnetic metal; and comprising a pole piece assembly made of a soft magnetic material surrounding said electrode.
According to yet another aspect of the present invention, there is provided a focusing/deflecting assembly for use in a charged particle beam column for inspecting a sample, wherein:
the focusing/deflecting assembly comprises a lens arrangement and a deflection system;
the lens arrangement comprises an objective magnetic lens and an electrostatic lens having an electrode made of a non-magnetic metal and formed with an opening for a charged particle beam passage therethrough, said electrode being accommodated downstream of the magnetic objective lens with respect to a direction of a charged particle beam propagation towards the sample, such that an optical axis of the lens arrangement intersects with said opening;
the deflection system comprises a magnetic deflector and a pole piece assembly having a portion made of a soft magnetic material which surrounds said electrode of the electrostatic lens.
The present invention, also provides according to its yet another aspect, a method of controlling a beam propagation through a lens arrangement of a charged particle beam column for inspecting a sample, the method comprising:
creating a magnetic deflecting field; and
conducting at least a portion of said magnetic field towards an optical axis of the lens arrangement.
The terms xe2x80x9cprimary beamxe2x80x9d and xe2x80x9cprimary charged particle beamxe2x80x9d used herein signify a charged particle beam, which is formed by charged particles generated by a source (cathode), and which is to be directed to a sample to knock out charged particles forming a xe2x80x9csecondary beamxe2x80x9d (also referred to as xe2x80x9csecondary charged particle beamxe2x80x9d), which is to be detected.
The charged particle beam may be an electron beam or a focused ion beam (FIB). The present invention may be used with an SEM or a similar tool applied to a specimen, e.g., a semiconductor wafer, for imaging, measurements, metrology, inspection, defect review or such purposes. For example, the present invention may be used for CD measurements, line profile measurements, copper-interconnects inspection/measurements, typically performed after a photolithography process, for automatic defect classification, etc.